Explanations

factory.img/.bin

initramfs

An initramfs and initrd are basically the same. It’s a filesystem in memory, which contains userland software. In an embedded environment it might contain the whole distro, on bigger systems it can contain tools&scripts to assemble&mount raid arrays and stuff like that before passing userland boot to them. Both can have a uHeader, to let uBoot know what it is.

initramfs-uImage.bin

initramfs-kernel.bin

sdcard.img.gz

sysupgrade / factory

There are basically two potential reasons why only a sysupgrade image is available for your router, either the router is already running some kind of OpenWrt fork (which understands the sysupgrade format natively) or there simply is no known way to assemble a factory image accepted by the router.Source: https://forum.lede-project.org/t/install-lede-on-sitecom-wlr-8100/3304/2

sysupgrade.bin/.tar

squashfs.bin/.chk/.trx

ubinized.bin

uImage

uImage is (in most cases) a zImage with a 64 byte uImage header, which contains the load address & entry point of the zImage, so that uBoot knows what to do with it. Further is contains a description of the actual contents (linux kernel, version, …)

zImage

zImage is a compressed plain kernel with a ‘pyggyback’. Some extra code which can decompress the kernel before booting it.